I am currently playing the game - so I hope I am answering your question and there isn't a
T-REx later that you are asking about - There is a tree in the area that has roots ABOVE
the ground. Run under the tree and stay there, the roots act as a cage and the T-Rex
can't get you. You can continue to shoot the T-Rex with total safety. Eventually, the
T-Rex will lay down and you can exit the area, wield your knife, and you will see a cut
scene that will show Turok killing the beast. Hope this helps.

same